This invention relates to powder coating compositions and to their use in coating substrates, especially substrates of complicated shape, with particular reference to articles having recessed portions.
Powder coating compositions generally comprise a solid film-forming resin binder, usually with one or more colouring agents such as pigments, and optionally also contain one or more performance additives. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer), but thermoplastic systems (based, for example, on polyamides) can in principle be used instead. Powder coating compositions are generally prepared by intimately mixing the ingredients (including colouring agents and performance additives) for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, for example by grinding, to the desired particle size. Other homogenisation processes also come into consideration, including non-extruder-based processes such as, for example, processes involving mixing using supercritical fluids, especially carbon dioxide.
Powder coating compositions are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate (normally metallic) is earthed. The charge on the powder coating particles is normally applied by interaction of the particles with ionised air (corona charging) or by friction (tribostatic or xe2x80x9ctriboxe2x80x9d charging). The charged particles are transported in air towards the substrate and their final deposition is influenced inter alia by the electric field lines that are generated between the spray gun and the workpiece. A disadvantage of this process is that there are difficulties in coating articles having complicated shapes, and especially articles having recessed portions, as a result of restricted access of the electric field lines into recessed locations (the Faraday cage effect), especially in the case of the relatively strong electric fields generated in the corona-charging process. The Faraday cage effect is much less evident in the case of tribostatic charging processes, but those processes have other drawbacks.
As an alternative to electrostatic spray processes, powder coating compositions may be applied by fluidised-bed processes, in which the substrate workpiece is preheated (typically to 200xc2x0 C.-400xc2x0 C.) and dipped into a fluidised bed of the powder coating composition. The powder particles that come into contact with the preheated surface melt and adhere to the workpiece. In the case of thermosetting powder coating compositions, the initially-coated workpiece may be subjected to further heating to complete the curing of the applied coating. Such post-heating may not be necessary in the case of thermoplastic powder coating compositions.
Fluidised-bed processes eliminate the Faraday cage effect, thereby enabling recessed portions in the substrate workpiece to be coated, and are attractive in other respects, but have the well-known disadvantage that the applied coatings are substantially thicker than those obtainable by electrostatic coating processes.
Another alternative application technique for powder coating compositions is the so-called electrostatic fluidised-bed process, in which the fluidising air is ionised by means of charging electrodes arranged in the fluidising chamber or, more usually, in the plenum chamber below the porous air-distribution membrane. The ionised air charges the powder particles, which acquire an overall upwards motion as a result of electrostatic repulsion of identically charged particles. The effect is that a cloud of charged powder particles is formed above the surface of the fluidised bed. The substrate workpiece (earthed) is introduced into the cloud and powder particles are deposited on the substrate surface by electrostatic attraction. No preheating of the substrate workpiece is required.
The electrostatic fluidised-bed process is especially suitable for coating small articles, because the rate of deposition of the powder particles becomes less as the article is moved away from the surface of the charged bed. Also, as in the case of the traditional fluidised-bed process, the powder is confined to an enclosure and there is no need to provide equipment for recycling and reblending the overspray that is not deposited on the substrate. As in the case of the corona-charging electrostatic process, however, there is a strong electric field between the charging electrodes and the substrate workpiece and, as a result, the Faraday cage effect operates to a certain extent and leads to poor deposition of powder particles into recessed locations on the substrate.
WO 99/30838 proposes a process which comprises the steps of establishing a fluidised bed of a powder coating composition, immersing the substrate wholly or partially within the said fluidised bed, applying a voltage to the substrate for at least part of the period of immersion, whereby particles of the powder coating composition are charged substantially by friction alone and adhere to the substrate, withdrawing the substrate from the fluidised bed and forming the adherent particles into a continuous coating over at least part of the substrate.
As compared with processes in which a substantial electric field is generated between charging electrodes and the substrate workpiece, the process of WO 99/30838, which is conducted without ionisation or corona effects in the fluidised bed, offers the possibility of achieving good coating of substrate areas which are rendered inaccessible by the Faraday cage effect.
The present invention provides a powder coating composition which incorporates a wax in a post-blended form.
The term xe2x80x9cpost-blendedxe2x80x9d means that the wax material has been incorporated after the extrusion or other homogenisation process (for convenience referred to hereinafter simply as xe2x80x9cextrusionxe2x80x9d).
The use of post-blended wax in accordance with the invention offers the possibility of achieving improved Faraday cage penetration in the coating of substrates and, as a result, more uniform coating of substrates having recessed areas or other locations rendered difficultly accessible by the Faraday cage effect, for example, the internal corner regions of microwave ovens. In particular, the invention enables the desired minimum coating thickness to be achieved on such regions without having to apply excess material to other more easily accessible areas of the substrate. Substantial savings of powder coating material are possible.
It will be understood that the use of post-blended wax in accordance with the invention is clearly distinct from prior proposals to incorporate wax for different purposes before or during extrusion. Such proposals can, however, be combined with the practice of the present invention.
The advantages of the invention are best seen in corona application processes, but other application processes may in principle be used instead, although the effect of the invention will generally then be less pronounced.
The invention further provides a process for forming a coating on a substrate, in which a composition according to the invention is applied to the substrate by a powder coating process, preferably a corona application process, resulting in particles of the composition adhering to the substrate, and forming the particles into a continuous coating.
The substrate is advantageously an article having recessed portions subject to the Faraday cage effect, and for an article having multiple faces the ratio of the minimum to maximum coating thickness is advantageously at least 40%, preferably at least 50%.
The invention also provides the use of a powder coating composition of the invention in coating an article having recessed portions which may be, for example, the interior of a refrigerator or microwave oven, an alloy wheel, an architectural extrusion or a radiator fin.
The wax in a powder coating composition of the invention is advantageously a synthetic wax, preferably a polyethylene (PE) or polytetrafluoroethylene (PTFE) wax, a PE wax modified with PTFE or polyamide, or a polyamide wax. In principle, however, other wax materials may be used instead, for example:
i) Natural animal waxes (for example, beeswax, lanolin);
ii) Natural vegetable waxes (for example, carnauba wax); or
iii) Natural petroleum or other mineral waxes (for example, paraffin wax, microcrystalline wax); or
iv) any of classes i)-iii) modified by PTFE or polyamide.
An important group of waxes for use in accordance with the invention comprises esters of long-chain aliphatic alcohols (typically C16 and above) with long-chain fatty acids (typically C16 and above). Such esters and acids are preferably straight-chain compounds, and may be saturated or unsaturated. Examples of acids which may be used include stearic acid, palmitic acid and oleic acid and mixtures of two or more thereof.
Waxes derived from long-chain aliphatic compounds as described above may include hydrocarbons.
In addition to esters of the long-chain acids as described above there may be mentioned salts such as, for example, aluminium stearate.
Preferred wax materials for use in accordance with the invention are materials which have good compatibility with the polymer component(s) of the powder coating composition, that is to say, materials which can be mixed homogeneously with the polymers without significant phase separation. It will be found that some wax materials (for example, halogenated waxes) are in general not compatible in this sense with the powder coating polymer(s). The use of such materials would be expected to give rise to defects in the surface appearance of the finished applied coating, and is accordingly not recommended.
Particular examples of suitable waxes include the following manufactured by Lubrizol: LANCO WAX A. 1601 (a fatty acid amide wax), LANCO WAX HM. 1666 (an amide wax) and LANCO WAX TF 1725 (a PTFE-modified polyethylene wax).
The amount of wax may be in the range 0.03-2%, but mention may be made of amounts in the range of from 0.03 to 0.8% by weight and 0.03 to 0.5% by weight. In addition, care is necessary to ensure that the powder coating composition does not become too sticky, and it may also be found that the penetration-enhancing effect of post-blended wax will diminish, with increasing wax addition, after a maximum value has been reached. The preferred maximum wax content will in general be 0.3 or 0.2%, more especially not exceeding 0.1%, all percentages being by weight and being based on the weight of the composition without the wax. Particular mention may be made of amounts in the range of from 0.05 to 0.1% by weight, especially 0.07 to 0.1%.
In general, the Tg of the wax should be above that of the remainder of the powder coating composition. This serves to reduce the tendency of the composition to become sticky as a result of incorporation of the wax. Preferably, the Tg of the wax is in the range of from 100xc2x0 to 140xc2x0 C.
In principle, more than one wax may be used as post-blended additive in accordance with the invention. In general, however, the use of a plurality of waxes will militate against the achievement of optimum results. If more than one wax is to be used, it is considered preferable to divide the base composition into a corresponding number of portions, post-blend a different wax with each portion and then mix the resulting powders together. Incorporation of two or more waxes in the same post-blending operation is not recommended.
Post-blending of the wax may be achieved, for example, by any of the following dry-blending methods:
a) tumbling the wax into the chip before milling;
b) injection at the mill, with the chip and wax fed into the mill simultaneously;
c) introduction at the stage of sieving after milling;
d) post-production blending in a xe2x80x9ctumblerxe2x80x9d or other suitable mixing device; or
e) introduction into a fluidised-bed powder reservoir supplying an electrostatic powder application gun.
In the case of method a) or b), the particle size of the wax is preferably less than that of the chip, and advantageously  less than 50 microns. In the case of method c), d) or e), the particle size of the wax is preferably less than that of the powder coating composition, preferably  less than 30 microns, more especially  less than 15 microns, for example  less than 10 microns.
The effects obtainable by the use of post-blended wax in accordance with the invention may be enhanced by the use, as further post-blended additives, of a combination of aluminium oxide and aluminium hydroxide, typically in proportions in the range of from 1:99 to 99:1 by weight, advantageously from 10:90 to 90:10, preferably from 30:70 to 70:30, for example, from 45:55 to 55:45. The combination of aluminium oxide and aluminium hydroxide is disclosed in WO 94/11446 as a fluidity-assisting post-blended additive. Other combinations of the inorganic materials disclosed in WO 94/11446 may in principle also be used in the practice of the present invention.
Such further post-blended additives may be incorporated with the composition simultaneously with the wax or separately from it, and may be incorporated by any of the post-blending techniques described in relation to the wax. Although any such additive or mixed sub-combination of additives may in principle be incorporated separately in the powder coating composition, pre-mixing of additives (other than the wax) is generally preferred.
Combinations of aluminium oxide and aluminium hydroxide (and similar additives) are advantageously used in amounts in the range of from 0.25 to 0.75% by weight, preferably 0.45 to 0.55%, based on the weight of the composition without the additives. Amounts up to 1% or 2% by weight may be used, but problems can arise if too much is used, for example, bit formation and decreased transfer efficiency.
Whilst the post-blended wax may in principle be in the form of wax deposited on a carrier material (such as, for example, silica), the use of such inhomogeneous materials is in general not recommended in the practice of the present invention.
The particle size distribution of the powder coating composition may be in the range of from 0 to 150 microns, generally up to 120 microns, with a mean particle size in the range of from 15 to 75 microns, preferably at least 20 or 25 microns, advantageously not exceeding 50 microns, more especially 20 to 45 microns. Although the invention can in principle offer advantages over the whole range of particle size distributions, it has been found that the benefits in terms of Faraday cage penetration tend to be less pronounced in relatively fine particle size distributions.
A powder coating composition according to the invention may contain a single film-forming powder component comprising one or more film-forming resins or may comprise a mixture of two or more such components.
The film-forming resin (polymer) acts as a binder, having the capability of wetting pigments and providing cohesive strength between pigment particles and of wetting or binding to the substrate, and melts and flows in the curing/stoving process after application to the substrate to form a homogeneous film.
The or each powder coating component of a composition of the invention will in general be a thermosetting system, although thermoplastic systems (based, for example, on polyamides) can in principle be used instead.
When a thermosetting resin is used, the solid polymeric binder system generally includes a solid curing agent for the thermosetting resin; alternatively two co-reactive film-forming thermosetting resins may be used.
The film-forming polymer used in the manufacture of the or each component of a thermosetting powder coating composition according to the invention may be one or more selected from carboxy-functional polyester resins, hydroxy-functional polyester resins, epoxy resins, and functional acrylic resins.
A powder coating component of the composition can, for example, be based on a solid polymeric binder system comprising a carboxy-functional polyester film-forming resin used with a polyepoxide curing agent. Such carboxy-functional polyester systems are currently the most widely used powder coatings materials. The polyester generally has an acid value in the range 10-100, a number average molecular weight Mn of 1,500 to 10,000 and a glass transition temperature Tg of from 30xc2x0 C. to 85xc2x0 C., preferably at least 40xc2x0 C. The poly-epoxide can, for example, be a low molecular weight epoxy compound such as triglycidyl isocyanurate (TGIC), a compound such as diglycidyl terephthalate condensed glycidyl ether of bisphenol A or a light-stable epoxy resin. Such a carboxy-functional polyester film-forming resin can alternatively be used with a bis(beta-hydroxyalkylamide) curing agent such as tetrakis(2-hydroxyethyl) adiparnide.
Alternatively, a hydroxy-functional polyester can be used with a blocked isocyanate-functional curing agent or an amine-formaldehyde condensate such as, for example, a melamine resin, a urea-formaldehye resin, or a glycol ural formaldehye resin, for example the material xe2x80x9cPowderlink 1174xe2x80x9d supplied by the Cyanamid Company, or hexahydroxymethyl melamine. A blocked isocyanate curing agent for a hydroxy-functional polyester may, for example, be internally blocked, such as the uretdione type, or may be of the caprolactam-blocked type, for example isophorone diisocyanate.
As a further possibility, an epoxy resin can be used with an amine-functional curing agent such as, for example, dicyandiamide. Instead of an amine-functional curing agent for an epoxy resin, a phenolic material may be used, preferably a material formed by reaction of epichlorohydrin with an excess of bisphenol A (that is to say, a polyphenol made by adducting bisphenol A and an epoxy resin). A functional acrylic resin, for example a carboxy-, hydroxy- or epoxy-functional resin can be used with an appropriate curing agent.
Mixtures of film-forming polymers can be used, for example a carboxy-functional polyester can be used with a carboxy-functional acrylic resin and a curing agent such as a bis(beta-hydroxyalkylamide) which serves to cure both polymers. As further possibilities, for mixed binder systems, a carboxy-, hydroxy- or epoxy-functional acrylic resin may be used with an epoxy resin or a polyester resin (carboxy- or hydroxy-functional). Such resin combinations may be selected so as to be co-curing, for example a carboxy-functional acrylic resin co-cured with an epoxy resin, or a carboxy-functional polyester co-cured with a glycidyl-functional acrylic resin. More usually, however, such mixed binder systems are formulated so as to be cured with a single curing agent (for example, use of a blocked isocyanate to cure a hydroxy-functional acrylic resin and a hydroxy-functional polyester). Another preferred formulation involves the use of a different curing agent for each binder of a mixture of two polymeric binders (for example, an amine-cured epoxy resin used in conjunction with a blocked isocyanate-cured hydroxy-functional acrylic resin).
Other film-forming polymers which may be mentioned include functional fluoropolymers, functional fluorochloropolymers and functional fluoroacrylic polymers, each of which may be hydroxy-functional or carboxy-functional, and may be used as the sole film-forming polymer or in conjunction with one or more functional acrylic, polyester and/or epoxy resins, with appropriate curing agents for the functional polymers.
Other curing agents which may be mentioned include epoxy phenol novolacs and epoxy cresol novolacs; isocyanate curing agents blocked with oximes, such as isopherone diisocyanate blocked with methyl ethyl ketoxime, tetramethylene xylene diisocyanate blocked with acetone oxime, and Desmodur W (dicyclohexylmethane diisocyanate curing agent) blocked with methyl ethyl ketoxime; light-stable epoxy resins such as xe2x80x9cSantolink LSE 120xe2x80x9d supplied by Monsanto; and alicyclic poly-epoxides such as xe2x80x9cEHPE-3150xe2x80x9d supplied by Daicel.
A powder coating composition for use according to the invention may be free from added colouring agents, but usually contains one or more such agents (pigments or dyes). Examples of pigments which can be used are inorganic pigments such as titanium dioxide, red and yellow iron oxides, chrome pigments and carbon black and organic pigments such as, for example, phthalocyanine, azo, anthraquinone, thioindigo, isodibenzanthrone, triphendioxane and quinacridone pigments, vat dye pigments and lakes of acid, basic and mordant dyestuffs. Dyes can be used instead of or as well as pigments.
The composition of the invention may also include one or more extenders or fillers, which may be used inter alia to assist opacity, whilst minimising costs, or more generally as a diluent.
The following ranges should be mentioned for the total pigment/filler/extender content of a powder coating composition according to the invention (disregarding post-blend additives):
0% to 55% by weight,
0% to 50% by weight,
10% to 50% by weight,
0% to 45% by weight, and
25% to 45% by weight
Of the total pigment/filler/extender content, the pigment content will generally be xe2x89xa640% by weight of the total composition (disregarding post-blend additives) but proportions up to 45% or even 50% by weight may also be used. Usually a pigment content of 25-35% is used, although in the case of dark colours opacity can be obtained with  less than 10% by weight of pigment.
The composition of the invention may also include one or more performance additives, for example, a flow-promoting agent, a plasticiser, a stabiliser against UV degradation, or an anti-gassing agent, such as benzoin, or two or more such additives may be used. The following ranges should be mentioned for the total performance additive content of a powder coating composition according to the invention (disregarding post-blend additives):
0% to 5% by weight,
0% to 3% by weight, and
1% to 2% by weight.
In general, colouring agents, fillers/extenders and performance additives as described above will not be incorporated by post-blending, but will be incorporated before and/or during the extrusion or other homogenisation process.
After application of the powder coating composition to a substrate, conversion of the resulting adherent particles into a continuous coating (including, where appropriate, curing of the applied composition) may be effected by heat treatment and/or by radiant energy, notably infra-red, ultra-violet or electron beam radiation.
The powder is usually cured on the substrate by the application of heat (the process of stoving); the powder particles melt and flow and a film is formed. The curing times and temperatures are interdependent in accordance with the composition formulation that is used, and the following typical ranges may be mentioned:
The invention is applicable over a wide range of applied film thicknesses, typically from thin films of, for example, 30 microns or less up to films of 50, 100, 150 or 200 microns. A typical minimum film thickness is 5 microns.
As a generality, for any given powder coating composition, the extent of advantage gained by the use of post-blended wax in accordance with the invention is dependent on the nature of the wax used. More specifically, it has been found in accordance with the invention that the results in terms of Faraday cage penetration can be enhanced by selecting the wax taking into consideration a measure of the tendency of the base composition to become positively or negatively charged in a tribocharging environment.
In one approach, mixtures consisting of one part which is a basic powder coating composition and another part which is the basic powder coating composition treated with a wax are charged tribostatically and the basic part is found to become charged predominantly in one sense while the wax-treated part is found to become charged predominantly in the opposite sense, permitting the separation of the mixture into the basic part and the wax-treated part by directing it at two oppositely charged plates. It is found that some mixtures of basic-part and wax-treated-part powder coating compositions separate to a greater extent than do others when directed at oppositely charged plates.
The fact that the basic-part and the wax-treated part of a powder coating composition are found to become oppositely charged provides a basis for establishing a triboelectric series of the powder coating compositions including basic powder coating compositions with and without wax treatment. The basic powder coating compositions themselves are known to be separable when mixed with one another and charged tribostatically, one basic powder coating composition acquiring a positive charge while the other acquires a negative charge, as shown by a tendency to separate onto two oppositely charged plates. In the resulting triboelectric series, the relative positions of the basic and wax-treated powder coating compositions are such that each powder coating composition takes on a negative charge in a charged mixture with the powder coating composition positioned immediately above it and a positive charge in a charged mixture with the powder coating composition positioned immediately below it.
The fact that some charged mixtures separate to a greater extent than do others leads to the expectation that basic and wax-treated powder coating compositions occupying widely separated positions in the triboelectric series separate from each other to a greater extent than do basic and wax-treated powder coating compositions that occupy adjacent positions in the triboelectric series.
A procedure for establishing a triboelectric series for the purposes of the present invention may include the following steps:
(i) selecting a plurality of powder coating compositions for inclusion in the triboelectric series,
(ii) selecting a first two of the powder coating compositions,
(iii) mixing the two selected powder coating compositions in substantially equal amounts,
(iv) causing tribostatic charging of the mixture of powder coating compositions by tribostatic interaction to establish equilibrium tribostatically charged conditions,
(v) directing the tribostatically charged mixture at two electrically charged plates of opposite polarities relative to each other,
(vi) identifying which of the two powders adheres to the electrically positive plate,
(vii) so allocating positions to the two powder coating compositions in the triboelectric series that the the powder coating composition which adheres to the positive plate occupies a position immediately below the position of the powder coating composition which adheres to the negative plate,
(viii) repeating the steps (ii) to (vii) until all of the powder coating compositions have been tested in pairs and allocated positions in the triboelectric series.
The steps (iv) and (v) above may be combined by ejecting the mixed powder coating compositions from a powder application gun supplied from a fluidised-bed hopper.
In a procedure which maintains the separation of the steps (iv) and (v) above, the step (iv) comprises placing two powders in a glass jar, shaking the glass jar for a set period, for example, about two minutes then allowing a 30 second relaxation time.
In a preferred procedure again maintaining the separation of the steps (iv) and (v) above, the step (iv) comprises fluidising the mixture and allowing it to develop its equilibrium natural tribostatic charge.
When the above procedure is performed on a plurality of coloured basic powder coating compositions visual identification of the basic powder coating compositions is permitted. Black powder coating compositions and white powder coating compositions may, of course, be included.
An adequate number of basic powder coating compositions for establishing a triboelectric series is seven and more than seven provides a more comprehensive triboelectric series. A minimum number of basic powder coating compositions for the triboelectric series is of the order of five. Specific materials may be included in the series in order to indicate reference positions although such materials are not necessarily included in powder coating compositions. Suitable reference materials are PTFE (polytetrafluoroethylene) occupying the lowest possible position and polyamide occupying the highest possible position in the triboelectric series.
The triboelectric series should include at least one pair of basic powder coating compositions which, when subjected to the above mixing, charging and separation procedure, separate between the charged plates to the extent that substantially all of one basic powder adheres to the positive plate and substantially all of the other basic powder adheres to the negative plate. Two such basic powder coating compositions fully satisfy the requirement for powder coating compositions that are well-separated in terms of triboelectric performance. Analogously, the triboelectric series will include basic powder coating compositions which, when subjected to the above mixing, charging and separation procedure, separate little or not at all between the charged plates. Two powder coating compositions that make up mixtures which separate little or not at all fail to meet the requirement for powder coating compositions that are well-separated in terms of triboelectric performance.
Where two differently coloured powder coating compositions are subjected to the above mixing, charging and separation procedure and the two powder coating compositions fully satisfy the requirement for powder coating compositions that are well-separated in terms of triboelectric performance, the result is that the colour of the powder coating composition adhering to the positive plate is substantially the same colour as one powder coating composition, the colour of the powder coating composition adhering to the negative plate being substantially the same as the colour of the other powder coating composition. It follows that a subjective quantitative assessment of the triboelectric performance of two differently coloured powders is possible by visual inspection of the colours of the powder coating compositions on the positive and negative plates relative to the respective colours of the powder coating compositions before they are mixed.
An objective quantitative assessment of of the triboelectric performance of two differently coloured powders is made with the assistance of a close tolerance reference colour spectrophotometer capable of operating in accordance with the CIE L*a*b*1976 system for assessing differences between colour samples. CIE is an abbreviation of Commission International d""Eclairage.
A suitable spectrophotometer is a Spectraflash SF600 PLUS CT manufactured by Datacolor International.
The CIE L*a*b*1976 system is a standard for defining colours in terms of a three-dimensional coordinate system and, for rectangular coordinates, a* is the x-coordinate variable, b* is the y-coordinate variable and L* is the z-coordinate variable. The range of L* is 0 to 100 and the ranges of a* and b* are both xe2x88x92100 to 100.
The following reference coordinates are included in the CIE L*a*b*1976 system:
Green: a*=xe2x88x92100, b*=0, L*=50
Red: a*=100, b*=0, L*=50
Blue: a*=0, b*=xe2x88x92100, L*=50
Yellow: a*=0, b*=100, L*=50
White: a*=0, b*=0, L*=100
Black: a*=0, b* =0, L*=0
The colour spectrophotometer operating in accordance with the CIE L*a*b*1976 system is capable of expressing the separation between two colour pigments as xcex94E, where xcex94E2=xcex94L*2+xcex94a*2+xcex94b*2 where xcex94L*, xcex94a* and xcex94b* are measured in the z, x and y directions, respectively. The magnitude of xcex94E is (xcex94L*2+xcex94a*2+xcex94b*2)1/2.
Elementary electrostatics permits the separation of oppositely charged particles by directing them towards oppositely charged plates. The negative particles are collected on the positive plate, and vice versa. Provided that there is some discernible difference between the two types of particle then the procedure permits the quantification of the degree of separation between two species in the mixture, by the use of differently coloured particles.
Established procedures for describing the charging behaviour of powder coatings use bulk measurements, which are relatively crude in assessing the charge characteristics of powders. By way of example, consider the following two cases:
A bulk charge measurement according to established procedures would be incapable of distinguishing between these two cases. As far as we are aware, there is no commercially available equipment for quantifying the charge distribution in powder coating compositions, so an indirect measurement of the charge behaviour must be made and that is achieved in accordance with the invention by the use of the parameter xcfx84 as explained hereinafter. The degree of charge separation in case A is substantially less than that in case B, and it has been found that the application of xcfx84 permits the selection of case B rather than case A as the mixture capable of showing the higher separation.
Quantification is most readily achieved in respect of two coloured powder coating compositions between which a significant xcex94E exists. A value of xcex94E (pure) between the pure powder coating compositions is first determined. The two powder coating compositions are then mixed in equal weight proportions, caused to become tribostatically charged and the charged mixture sprayed through a powder delivery gun at two oppositely charged plates, resulting in a degree of separation of the two powder coating compositions on to the two charged plates according to the relative charges acquired by the two powder coating compositions. The tribostatic charging, preferably, includes fluidising the mixture and allowing it to develop its equilibrium natural tribostatic charge. After suitable treatment, for example stoving, causing the powder coating compositions to become fixed to the two plates, a value xcex94E (mixture) is determined between the powder coating compositions on the two plates.
In accordance with the invention a parameter xcfx84 has been developed as a practical tool in the assessment of the triboelectric performance of two differently coloured powders using the parameter xcex94E. The parameter xcfx84 is defined as xcfx84=xcex94E(mixture)/xcex94E(pure). xcex94E(pure) indicates a value for xcex94E between two pure powders. The determination of xcex94E (mixture) comprises mixing the two powders in about equal weight proportions, causing the charging of the resulting mixture by tribostatic interaction to establish equilibrium tribostatically charged conditions, preferably by fluidising, and causing the mixture to separate by spraying it through a powder delivery gun with no applied voltage at two oppositely charged plates, xcex94E(mixture) being the value of xcex94E between the xe2x80x9cseparatedxe2x80x9d mixture distributed on the oppositely charged plates.
It has been found that the use of colour information permits practical quantification of the extent to which tribostatically charged powder particles separate and that the results of colour measurements are of practical value in the selection of highly-separating powder mixtures.
Preferably, a powder coating composition is characterised by a triboelectric interaction factor xcfx84, between the composition incorporating the wax and the same composition without the wax, of xe2x89xa70.25, xe2x89xa70.3, xe2x89xa70.4, xe2x89xa70.5, xe2x89xa70.6, xe2x89xa70.7 or xe2x89xa70.8, the value of xcfx84 being given by the relationship
xcfx84=xcex94E(composition mixture)/xcex94E(pure compositions) 
where
xcex94E=(xcex94L*2+xcex94a*2+xcex94b*2)1/2 
with L*, a* and b* being respectively the z, y and x- coordinate variables under the CIE L*a*b*1976 colour definition system,
xcex94E (pure compositions) being determined by colour spectrophotometric measurement and xcex94E (composition mixture) being determined by mixing the two compositions in equal weight proportions, causing the charging of the resulting mixture by tribostatic interaction to establish equilibrium tribostatically charged conditions, directing the charged mixture onto two oppositely charged plates, resulting in a separation of the compositions on the two plates, and then determining xcex94E, by colour spectrophotometric measurement, between the compositions as applied to the two plates, one or both of the respective initial pure compositions being dyed where appropriate to provide an enhanced xcex94E between them to facilitate the determination of xcex94E (pure compositions) and xcex94E (composition mixture).
The ratio xcfx84=xcex94E (mixture)/xcex94E (pure) is attributed to the mixture of the two powders. If, say, there has been total separation of the powder coating mixture between the two plates, then xcex94E (mixture) would be the same as xcex94E (pure) and the ratio xcfx84 would have a value of 1, possibly giving the same result as a subjective visual examination of the two plates. If, on the other hand, there has been no separation of the powder coating compositions between the two plates, the two plates would be of substantially the same colour and xcex94E (mixture) would be substantially zero, leading to a ratio xcfx84=0, which might be determined by visual inspection of the two plates. The ratio xcfx84 can, of course, be found to attain any value between 0 and 1, both limits included, according to the value of xcex94E (mixture) between the powder coating compositions adhering to the plates in relation to xcex94E between the pure powder coating compositions.
A modified form of the above procedure is applied in the case of two coloured powder coating compositions between which there is not a significant xcex94E and, also, in the case of two white powders. The modification involves the addition of a first dyestuff to one powder and, where appropriate to provide an enhanced xcex94E, the addition of a second dyestuff to the other powder coating composition, the added dyestuffs being such as not to influence the relative charges acquired by the powder coating compositions. The dyestuffs are so selected as to have a significant xcex94E and the remainder of the procedure set out above is followed in order to obtain xcex94E for the mixture of the two powder coating compositions. Following the addition of the dyestuffs, each dyed powder coating composition should be checked in relation to the triboelectric series in order to be sure that the addition of the dyestuff does not result in a change in the position of the powder coating composition in the triboelectric series.
Dyestuffs may also be used for determining the triboelectric performance of two white powder coating compositions following a check, as before, that the addition of the dyestuff does not cause a change in the position of either powder coating composition in the triboelectric series.
The value xcex94E when used in the calculation of xcfx84 is considered to give accurate enough results for practical purposes although the use of xcex94L*, xcex94a* and xcex94b* would be expected to provide more accurate determinations of xcfx84.
It has been found that a value for xcex94E of 2 is large enough to give satisfactory reproducible results in the determination of xcfx84.
Values of xcfx84 greater than 0.25 have been noted to result in enhanced penetration of a mixture of powder coating compositions compared with the penetration of the respective powders into recesses, a value of xcfx84 greater than 0.5 is preferred and a value of T greater than 0.6 is especially preferred. More generally, the value of xcfx84 may be xe2x89xa70.3, xe2x89xa70.4xe2x89xa70.5, xe2x89xa70.6, xe2x89xa70.7 or xe2x89xa70.8.
In the case of white powder coating compositions or coloured powder coating compositions showing not much difference in xcex94E, the triboelectric performance may be quantified, alternatively or additionally, by incorporating a small amount of two heavy metal compounds into the respective powder coating compositions and measuring the relative amounts of the heavy metal compounds in the powder coating compositions after mixture and separation on to oppositely charged plates. The measurement would be by means of X-ray fluorescence spectroscopy or X-ray mass analysis using a scanning electron microscope.
For a mixture of a white basic powder coating composition and the white powder coating composition treated with a wax, a dyestuff is added to the wax-coated part, say, before the two parts are mixed, as a means of making it possible to determine T for the mixed basic-part and wax-treated part powder coated composition. A red dyestuff is chosen, added to a body of the basic powder coating composition and the basic-without-dyestuff and basic-with-dyestuff powder coating compositions compared in accordance with the CIE L*a*b*1976 system to determine xcex94E(pure). Since red has coordinates of L*=50, a*=100, b*=0 in the CIE L*a*b*, 1976 system, xcex94E (pure)=xcex94a*(pure) provided xcex94b and xcex94L are zero. An amount of the basic-with-dyestuff powder coating composition is then treated with a specific amount of a selected wax, the basic-with-dyestuff-with-wax and the basic powder coating compositions are mixed together, caused to become tribostatically charged, separated on to positive and negative plates and xcex94a*(mixture) is measured to give xcfx84(mixture)=xcex94a*(mixture)/xcex94a*(pure).
The red dyestuff may be substituted by a green dyestuff, in which case, for the above procedure, xcfx84=xcex94a* (mixture)/xcex94a* (pure), since green has coordinates L*=50, a*=xe2x88x92100, b*=0 and again xcex94E=xcex94a* provided xcex94b and xcex94L are zero.
Dyestuffs may also be used with coloured powder coating compositions in order to determine xcfx84 for basic-part and wax-treated part mixtures of those powder coating compositions.
The proportion of the dyestuff needed, that is, the proportion needed to achieve xcex94Exe2x89xa72, will in general be xe2x89xa60.4% by weight, although usually a lower proportion will suffice, say, of the order of 0.1%.
Having established a triboelectric series as described above, a corresponding determination is then made of the position in the series of the powder coating composition actually to be used for a given application ( which may be a white or a coloured powder), hereinafter the xe2x80x9cend user powder.xe2x80x9d
Advantageously, in the practice of the present invention, a wax is selected on the basis of the information provided by the triboelectric series to provide basic end user and wax-treated end user powder coating compositions which are separated in the triboelectric series (in either the positive or negative direction) and, preferably, the basic end user and the wax-treated end user powder coating compositions are widely separated in the triboelectric series.
Preferably, the separation between the basic end user and wax-treated end user powder coating compositions as assessed by the above method using the CIE L*a*b*1976 system gives a xcfx84 of more than 0.5 and, preferably, more than 0.6.
The position of any given powder coating composition in the triboelectric series may in principle be influenced by a number of variables, including:
(a) the nature and amount of any colouring agent (pigment or dye);
(b) the nature and amount of any filler/extender;
(c) the nature and amount of any post-blended additive;
(d) the use of a tribo-enhancing additive known from conventional tribostatic application to enhance the tribostatic performance such as, for example, an amino alcohol or a tertiary amine or other suitable pre-extrusion additive.
The effect of altering any of the above variables can be determined by routine experimentation.